Yeast and human cells require the ESS1 gene in order to divide. This essential gene controls cell cycle progression through mitosis, and given its extraordinary conservation, is probably required in all eukaryotes. The goal of this work is to understand how ESS1 controls the cell cycle. The yeast saccharomyces cerevisiae, the organism in which ESS1 was discovered, will be used as a model system. ESS1 is highly conserved both structurally and functionally; ESS1 homologs from Drosophila melanogaster and humans rescue yeast ess1 mutants. The Ess1 protein and its counterparts are the only known proteins to contain both a WW domain and a PPIase (peptidyl-prolyl isomerase) domain. The WW domain is a protein-protein interaction module found in proteins such as Dystrophin, encoded by the human muscular dystrophy gene. The PPIase domain is a catalytic module that controls the folding and activity of kinases, receptors and transcription factors and is found in proteins like cyclophilin that mediate the effects of immunosuppressive drugs. How these domains function in Ess1 to control the cell cycle is not known. In this proposal, yeast genetics will be used understand how Ess1 controls mitotic progression and how the WW and PPIase domains contribute to this activity. This will be accomplished by (1) conducting an extensive structure- function analysis of the Ess1 WW and PPIase domains, (2) by isolating conditional alleles of ESS1 gene and using these alleles for cell cycle analysis, and (3) by identifying other components of the ESS1 pathway using suppressor screens and tow-hybrid selections. An understanding of how ESS1 functions in yeast will illuminate poorly understood pathways of mitotic regulation and will provide insight into the control of the cell cycle in all eukaryotic cells.